Leukaemia is a type of cancer in which there is abnormal increase in the number of

The correct answer is D (white blood cells). Leukaemia is a cancer of the blood-forming tissue in which the bone marrow produces abnormal, immature white blood cells (leucocytes) in huge numbers. These cells crowd out normal red cells, platelets and healthy white cells, which is why anaemia, bleeding and infection are the typical features.

Why the others are wrong:

• A (bone cells): Cancers of bone are sarcomas such as osteosarcoma and Ewing's sarcoma. Leukaemia starts in the marrow, but the cells involved are blood-cell precursors, not bone cells.
• B (platelets): An abnormal rise in platelets is thrombocythaemia, a myeloproliferative disorder, not leukaemia.
• C (red blood cells): An excess of red cells is polycythaemia (polycythaemia vera), again a distinct condition.

The core concept is the classification of blood cancers: leukaemia (white cells in marrow and blood), lymphoma (lymphoid tissue) and multiple myeloma (plasma cells). For Mains GS-III, the rising cancer burden and the National Cancer Grid are a natural link.

Trap: Aspirants read "bone marrow" and pick A, or associate blood disorders with platelets and pick B. The question asks which cell type increases, and the answer is white cells.